High efficiency air filtration media is used in commercial and industrial air cleaning equipment. An example of such media is the "VARICEL.RTM." air filter marketed by American Air Filter Co., of Louisville, Ky. These filters provide excellent particulate matter removal from the air that passes through them without sacrificing pressure drop. These types of filter, however, are made with glass fibers or glass microfibers (referred to hereinafter as "glass fibers"). These glass fibers are suspected of being cancer causing agents and the air filtration industry is looking at ways of eliminating the use of glass fibers in air filtration media. See: "Cancer Risk Cited in Three Man-Made Fibers", Washington Post, Jun. 27, 1987, p. f04; "Cancer Puts Fiberglass Under the Microscope", Wall Street Journal, Eastern (Princeton, N.J.) Edition, Jun. 17, 1991, page B1; Kilburn, K. H., et al., "Difficulties of Attribution of Effects in Workers Exposed to Fiberglass and Asbestos", The American Journal of Industrial Medicine, Vol. 20, No. 6, (1991) pps. 745-751; "Fiberglass Not Carcinogenic, OSHA Says", Environmental News Networks Inc., Global Warming Network-GWN Online Today, Jul. 26, 1991; and Saracci, R., "Ten Years of Epidemiologic Investigations On Man-Made Mineral Fibers and Health", Scandinavian Journal of Work Environment and Health, 12 Supplement No. 1, (1986) pps. 5-11.
Attempts to replace glass fiber-based air filtration media with melt-blown fabrics have met limited success. While these melt-blown fabrics can provide high particulate matter removal efficiencies, they suffer by being light-weight, weak, and limp. Such materials produce structurally deficient filters. Accordingly, the use of melt-blown fabrics, at this time, does not appear to be a viable option.
Finally, filter media utilizing various combinations of staple fibers, fibrids, and heterofils are known. See, for example, U.S. Pat. Nos. 4,160,059; 4,274,914; 5,022,964; and pending U.S. patent application Ser. No. 07/999,031, filed Dec. 31, 1992, each of which is incorporated herein by reference. U.S. Pat. No. 4,160,059 is directed to nonwoven fabric produced by a dry process consisting essentially of: (1) supporting fibers which consist of (a) natural and/or synthetic fibers and (b) heat fusible fibers; and (2) an adsorptive material. U.S. Pat. No. 4,274,914 is directed to a filter sheet comprising cellulose ester staple fibers, cellulose ester fibrids, and excluding any binder. U.S. Pat. No. 5,022,964 is directed to a nonwoven fibrous web that contains cellulose ester fibers, cellulose ester fibrids, and a minor amount of activated fusible fibers (binder fibers). U.S. application Ser. No. 07/999,031 filed Dec. 31, 1992 is directed to a wet-laid filter structure comprising bicomponent fibers (heterofils) and may include microfibers.